Research work concerning vertical mixing processes in stratified water areas is briefly reviewed. Attention is focused on vertical mixing phenomena which are found in the hydraulic engineering field, such as estuaries, lakes, reservoirs, land locked bays, etc. From the view point of the mixing mechanism, entrainment processes are classified into three categories, i.e., (1) shear induced entrainment (Stype), (2) entrainment due to eddy convection and diffusion in the mechanical stirring fied (DM-type) and (3) entrainment due to thermal convection (DT-type). The turbulence structure and entrainment mechanism for each case are examined and discussed. Experimental and analytical work on entrainment relationships is shown for several types of stratified flow fields among which shear-free turbulence generated by an oscillating grid, stratified shear flows, wind-induced mixing and turbulent penetrative convection are included.
In the case of collecting grains and removing muddy deposits, especially those containing contaminants, it is very difficult to lift them up effectively by means of suction alone. Our investigations revealed that effective and smooth collection and transportation can be performed by mounting an injector at the collecting mouth in order to fluidize the settled bed. A sand collector with an injector at the collecting mouth can be applied not only for gases but also for liquids as a carrier and can transport many kinds of grains, such as sand and other relatively small particles. Several variables affect the performance of this device. In this paper, the fundamental characteristics of the system and its application are reviewed.
Circulation characteristics and flow instabilities are investigated in a natural circulation loop of liquid nitrogen. The circulation velocity increases with the increase in the heat flux and decreases with the increase in the flow restriction. Beyond a certain limit of heat flux, so called “density wave oscillation” occurs. By applying a lumped-parameter model and homogeneous two-phase flow model to the present natural circulation system, the numerical simulation is conducted. The simulation well represents the transient behavior of the system and the estimated threshold conditions of the flow oscillation agree well with the experimental results.
Air lift pumps are regarded as one of the useful means of conveying liquid or solid particles. Accurate methods for estimating pump performance are required to carry out rational design of the pumps. Although several numerical methods to predict the pump performance have been already proposed, none of them are based on the multi-fluid model. Hence, a numerical method based on the multi-fluid model for steady gas-liquid-solid three-phase flow was proposed and examined in the present study. Since it had not been clear if the multi-fluid model could be applied to air lift pumps, pumps for lifting liquid were analyzed by implementing well-known correlations of the two-fluid model into the present method. It was confirmed by comparisons between calculated and available experimental data that the present method can predict the pump performance accurately under a wide range of flow conditions. In other words, the validity of the present method and the adopted correlations was confirmed. Then, a pump to lift solid particles was analyzed. Since there are few available correlations for the three-fluid model, correlations adopted in the analysis were tentative ones. However the present method gave relatively good predictions.